Power pentodes as preamp tubes?

[Alan0354]

Not to rain on your parade, but 6L6 (or any other power pentode) transconductance at, say, 1mA plate current (you are feeding it through 100K after all) is WAY lower than at usual power tube current levels.
You want a very rough, no graphs voltage gain estimation for a 6L6 ? (EL34/84 can be calculated in a similar way):
Just consider this:
Idle: plate voltage=V+ , say, 430V ; grid voltage (bias)= -52V
Saturating: Plate, say, 60V ; grid: 0V .
(Delta means "variation")
Plate variation (delta voltage)= 430-60=370V
Grid variation (delta voltage) = 52-0=52V
Effective gain: 370/52=7 ..... incredible, isn't it?

So, what about that huge transconductance, is it a lie? Not at all, but is is the name to describe a *type* of Physical phenomenon: that when voltage changes, current changes.
In this case , it does not mean huge voltage gain but huge current swing for a relatively small (grid) voltage swing.

So, in a nutshell, use small triodes (or small pentodes if you wish) for gain and big pentodes for power.
Can't go wrong.

Hasn't been wrong for the last 90 years, go figure

That's what I was worry about, that's why I did mention whether you get any gain in this low current level as all the spec are for running at optimal current and load.

[Merlinb]

Still back to my original, how about the grid current. I have no data on this, this cause noise. Common sense tells me that the grid of a big pentodes is going to be a lot bigger, if there is any electrons from cathode or plate or others hitting the grid, you get more surface area and therefore more grid current. That will raise the noise. If you have a high gain design, that might give you a lot of hiss.

Grid current is negligible as long as the brid is biased negative, which is normally the case for audio amps. You can safely ignore noise due to grid current- all that matters is shot and flicker noise in the anode current.

[Alan0354]

Grid current is negligible as long as the brid is biased negative, which is normally the case for audio amps. You can safely ignore noise due to grid current- all that matters is shot and flicker noise in the anode current.

It all depend, the spec of the grid 1 impedance of 100K in fixed bias and 500K in cathode bias of 6L6GC implies you cannot count on it's like an open circuit. Also it is explained in RDH4 about the electrons hitting the grid from cathode and gas etc. That's current. If people worry about thermal noise of a 100K grid stopping resistor, you better look into this. 1uA grid current is not small by any stretch. We worry about bias current of 1nA or smaller in low noise designs.

[Alan0354]

Grid current is negligible as long as the brid is biased negative, which is normally the case for audio amps. You can safely ignore noise due to grid current- all that matters is shot and flicker noise in the anode current.

It all depend, the spec of the grid 1 impedance of 100K in fixed bias and 500K in cathode bias of 6L6GC implies you cannot count on it's like an open circuit. Also it is explained in RDH4 about the electrons hitting the grid from cathode and gas etc. That's current. If people worry about thermal noise of a 100K grid stopping resistor, you better look into this. 1uA grid current is not small by any stretch. We worry about bias current of 1nA or smaller in low noise designs. Better get the grid current spec first.

[Austin]

So I should keep something like 30 ma through the tube to keep the transconductance up then?

Also the 100k grid stopping resistors are alot bigger than I intend to use. Why shouldn't I use smaller less noisy values so a bit of grid current wouldn't be a problem? I am not sure but it makes sense that the grid of power tube has some grid current even while at less potential than the cathode if even the 12ax7 does.

[Austin]

[Steve Conner]

Any parameter not specified on the datasheet will do whatever it takes to ruin your day. That's one of the corollaries of Murphy's Law of electronic design.

It follows that power tubes used as preamp tubes are likely to be somewhat noisy, probably for reasons similar to what Alan0354 set out. Noise figure wasn't on the datasheet, so the factory wouldn't have had quality control in place for it.

I say this with a hint of sadness as I've been looking for discrete transistors specified for low noise in the audio band, but I can't find any in current production, except for the high priced dual arrays, MAT01 and so on.

[Austin]

It won't hurt to try and see what happens I spose.. I'm cobbling something up right now....

[Steve Conner]

Whoa, I can feel the vintage mojo right there!

[Alan0354]

Any parameter not specified on the datasheet will do whatever it takes to ruin your day. That's one of the corollaries of Murphy's Law of electronic design.

It follows that power tubes used as preamp tubes are likely to be somewhat noisy, probably for reasons similar to what Alan0354 set out. Noise figure wasn't on the datasheet, so the factory wouldn't have had quality control in place for it.

I say this with a hint of sadness as I've been looking for discrete transistors specified for low noise in the audio band, but I can't find any in current production, except for the high priced dual arrays, MAT01 and so on.

Yes, I gone through looking, the price of MAT01 is shocking to put it politely. I did a little search before, for low noise, high beta and low cost, MPSA18 for NPN and 2N5087 for PNP is hard to beat.

[Alan0354]

Any parameter not specified on the datasheet will do whatever it takes to ruin your day. That's one of the corollaries of Murphy's Law of electronic design.

It follows that power tubes used as preamp tubes are likely to be somewhat noisy, probably for reasons similar to what Alan0354 set out. Noise figure wasn't on the datasheet, so the factory wouldn't have had quality control in place for it.

I say this with a hint of sadness as I've been looking for discrete transistors specified for low noise in the audio band, but I can't find any in current production, except for the high priced dual arrays, MAT01 and so on.

Yes, I gone through looking, the price of MAT01 is shocking to put it politely. I did a little search before, for low noise, high beta and low cost, MPSA18 for NPN and 2N5087 for PNP is hard to beat. If you have better ones, let me know.

You need high beta to lower the base current Ib.....which lower the shot noise.

Then the FETs which has the advantage in noise performance in RF, sucks in audio frequency as the 1/f noise is going to swamp all the other sources. Still the good old tried and true BJT.

In fact, I am waiting for 4 of my pcb coming back, those are for pedals that I have been working on. I came here to talk about tubes few months ago and totally dropped it. It was only because last month my Marshall JCM900 blown!! $#X&!!! I have no choice but to talk out my old beat up Fender Bassman 100 and rip out the whole front end and designed my own clean and dirty channel in. That's got me started again on the tube stuff. I like the result which promp me to start looking into the power amp. So far all I did was lower the grid voltage to bring the power tube more towards class A by drawing more quiescent current. Then I changed out the big OT with a Magnetic Components 40W OT for a pair fo 6L6GC instead of 4.

[Alan0354]

This is a good explanation of grid current:

http://music-electronics-forum.com/t30481/

[Austin]

This is a good explanation of grid current:

http://music-electronics-forum.com/t30481/

Where do you guys think grid current goes when it leaves the driver and goes into the grid of the next tube?

[exclamationmark]

I'm fairly sure it just sums with the plate current and can be measured through the cathode. I recall measuring the characteristics of some 6AS7G tubes and wondering why the voltage was different (not a huge difference but still significant) across each respective 1 ohm plate and cathode resistor when the grid was driven hugely positive. The DC cathode follower arrangement found in a ton of amps also seems to support this, as the cathode follower 'steals' current from the previous stage to achieve proper bias.

[Merlinb]

MPSA18 for NPN and 2N5087 for PNP is hard to beat. If you have better ones, let me know.

BJT noise at LF is also dependent on base spreading resistance. Devices rated for high collector currents usually have smaller spreading resistance. Obviously there is a balance to be struck between minimising base current (implying high hFE) and in minimising spreading resistance (implying a high current rating and therefore probably low hFE). The ZTX653/753 and BC337/327 represent good examples, as they both have low spreading resistance and healthy hFE >200.